#include<iostream>
using namespace std;
int main()
{
	int k = 0, sum = 0, i = 0, d = 1, m = 1;
	cin >> k;
	for (i = 1; i <= k; i++)
	{
			sum += m;
			d--;
			if (d == 0)
			{
				m++;
				d = m;
			}
	}
	cout << sum << endl;
	return 0;
}